1. Field of the Invention
The present invention relates to an engine control device that uses an intake air amount, which is a control parameter of an engine, namely an internal combustion engine (hereafter, referred to simply as engine), after correcting the intake air amount by humidity.
2. Description of the Related Art
Recent years have witnessed the spread of engine control devices of so-called “torque base control” type that control the torque generated by an engine using, as an index, an engine output shaft torque that is utilized as a requested value of driving force from the driver or from the vehicle. In such a torque base control, a target torque of the engine is determined on the basis of an operation amount of an accelerator pedal by the driver. A throttle opening degree is then controlled in such a manner that a target intake air flow rate that allows generating the target torque is sucked into the engine, and a fuel injection amount or ignition timing is controlled in accordance with an actual intake air flow rate such that engine output is controlled to the target torque, whereby the running performance requested by the driver is realized.
In such an engine control device that realizes the target intake air flow rate corresponding to the target torque of the engine, throttle opening is controlled through driving of an actuator that is linked to the throttle of the engine. Specifically, a technology has been proposed wherein a target opening area of the throttle is worked out by being applied to a flow rate calculation expression or equation of a diaphragm type flow meter based on a target intake air flow rate, a pressure ratio before and after the throttle, a throttle opening area and so forth, and an actuator linked to the throttle is controlled so as to bring about a throttle opening at which the target opening area of the throttle is attained.
The amount of air that is sucked into the engine is detected by an air flow sensor (hereafter, occasionally abbreviated as “AFS”) that is disposed in an air intake passage. As the AFS, there is generally used a thermal fluid sensor in which the amount of heat transferred to a fluid from a heating element disposed in the fluid is detected electrically, to detect thereby the flow rate of the fluid (for instance, Japanese Patent No. 5680178).
In the environment where the engine is used, water vapor denoted by humidity is generally present in the air that is sucked into the engine. Humidity, i.e. the amount of water vapor in the air, varies depending on weather conditions and the like. The heat transfer rate and viscosity coefficient of “moist air”, which is air that contains water vapor, varies depending on the amount of water vapor comprised in the moist air. The amount of heat transferred to the moist air from a heating element that is disposed in the AFS varies depending on the water vapor amount, for an identical air amount. This gives rise to air amount measurement errors in the thermal fluid sensor.
As a technology for correcting the influence of humidity on the air amount detected by the thermal fluid sensor, i.e. on the detected value of moist air amount, a circuit configuration has been proposed in which a moisture-sensitive resistive element for air humidity compensation is provided in an electronic circuit for air flow rate measurement that has a heating resistor of a thermal fluid sensor, to perform humidity correction on a measured value of air flow rate (for instance, Japanese Patent No. 2957769).
However, engine output in gasoline engines or the like is adjusted according to the amount of air that is supplied to the cylinders. The air amount is adjusted through adjustment of the opening degree of a throttle valve that is provided in an air intake passage to the cylinders. A fuel such as gasoline is mixed with the air that is supplied to the cylinders. The air-fuel mixture is compressed by pistons and is ignited. The increment in pressure derived from combustion of the air-fuel mixture is the output of the engine. Within the amount of air that is sucked into the cylinders, only dry air, which excludes water vapor comprised in air, undergoes combustion.
In Japanese Patent No. 2957769, therefore, a difference arises in the output of the engine depending on the humidity comprised in moist air i.e. depending on the amount of water vapor, even if the detected moist air amount stays the same.
An air flow rate measuring device has also been proposed in which a dry air amount is calculated from information on temperature and humidity, and the dry air amount is output as an air amount after humidity correction (for instance, Japanese Patent Application Publication No. H10-2772).